Some electronic systems use differential signaling to improve speed and noise immunity. A pair of differential signals has one signal line driven high when the other signal line is driven low to transmit a bit of data. The difference in voltage between the two signal lines in the differential pair, rather than the absolute voltages, determines the data state.
Failures can occur, such as when a cable carrying a differential pair breaks or is accidentally disconnected. Fail-safe circuits are sometimes added to differential inputs to detect when such a cable break occurs. Often a load resistor at the receiver side of a cable connects the two signal lines in the differential pair.
When the cable is disconnected, the load resistor equalizes the voltages on the two signal lines until both signal-line inputs to the differential receiver have the same voltage. This same-voltage condition is detected by the differential receiver, and the output of the differential receiver is forced to a known state, rather than left in an unstable or undefined state.
Rather than simply force the differential receiver output to a known state, detection of equal voltages on the differential input signal lines can be used to power-down a circuit or sub-system. See for example, “Power Down Mode Signaled by Differential Transmitter's High-Z State Detected by Receiver Sensing Same Voltage on Differential Lines”, U.S. Ser. No. 10/064,074, filed Jun. 7, 2002, now U.S. Pat. No. 6,593,801, and “Power-Down Activated by Differential-Input Multiplier and Comparator”, U.S. Ser. No. 10/249,280, filed Mar. 27, 2003, now U.S. Pat. No. 6,791,371, both assigned to Pericom Semiconductor Corp. of San Jose, Calif.
While such equal-voltage detectors for power-down control are useful, a more advanced detector is desired for detecting the equal-voltage condition on a pair of differential lines. An advanced equal-voltage detector and power-down circuit is desired.